In general, a bill processing apparatus, which handles a bill as one of the embodiments of the paper sheet, is incorporated into a service device such as a game medium rental machine installed in a game hall, a vending machine or a ticket-vending machine installed in a public space, or the like which judges the authenticity of the bill inserted from a bill insertion slot by a user and provides various types of products and services in accordance with a value of the bill having been judged as authentic.
Generally, an authenticity judgment of a bill is conducted by irradiating the bill moving along the bill travelling route with light, having a light receiving sensor detect the transmitted light and reflected light from the bill, and comparing the detected data with reference data stored in dictionary data. The authenticity judgment process is performed by extracting various characteristics of a bill, and it is known that the authenticity judgment is conducted, as an example, by detecting the length of the bill.
For example, Patent Reference 1 discloses a method of judging an authenticity of a bill by determining a length of the bill, while a conveyor roller (conveyor member) to convey a bill, a pulse motor to rotationally drive this conveyor roller, and a sensor to detect the bill being conveyed along a traveling route are provided, as a product of a conveying distance per one pulse and a pulse number of the pulse motor counted during a period in which the sensor detects the bill is obtained.    [Patent Reference 1] Japanese unexamined patent application publication No. H5-12527